Method for preparing addition type polyimide prepregs

ABSTRACT

A novel addition polyimide based on the use of liquid monomers wherein the essentially solventless prepreg produced therefrom retains good drape, tack and other mechanical properties.

ORIGIN OF THE INVENTION

The invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

This is a division of application Ser. No. 839,963, filed Oct. 6, 1977,now U.S. Pat. No. 4,166,170 issued Aug. 28, 1979.

BACKGROUND OF THE INVENTION

The concept of depression of melting points by mixing two or more purecomponents is well known in the fields of alloying metals and organicchemistry. For example, when an organic material having a melting pointof 200° C. is mixed in a 50:50 ratio with another organic materialhaving a melting point of 175° C., the resulting mixture may very wellhave a melting point of 90° C. This effect is often attributed to theinteraction or mixing of the two components in the liquid state thusmaking it difficult for either of the constituents to assume theirpreferred crystalline or solid state. When considering organiccompounds, the ones that tend to form very thermodynamically stablecrystals experience the greatest depression in melting point especiallywhen mixed with a compound having a different crystalline structure. Forhigh molecular weight polymers, this depression is not very pronouncedunless the polymer is crystalline. The family of polymers known aspolyimides are well known to be amorphous and by mixing two differentpolyimides you would therefore not expect a significant lowering ofeither the softening or melting points.

One approach to preparing polyimides that does lend itself to thelowering of melting point concept is via the addition polymerizationroute. This process involves the preparation of an oligomeric imidehaving vinyl endcaps and generally represented by the formula: ##STR1##and where R is selected from aromatic amines such as ##STR2## where n=0,1, 2, 3 . . .

When an oligomer of this type is heated it generally melts andpolymerizes at the proper temperature through the vinyl linkage.##STR3## The resultant polymeric material is highly crosslinked and istherefore thermoset. By polymerizing this type of material "in place" itis useful as an adhesive. This same processing method must be used forthe preparation of fiber reinforced composites where the polymerizationencapsulates the reinforcement and is set. Also this process is usedwhereby epoxies serve as adhesives and for composite matrix resins.

One particularly attractive addition-type polyimide that is commerciallyavailable is Ciba Geigy's P-13N which has the following structure:##STR4## where n=1 or 2.

When this material is heated at 550°-600° F., the imide ring forms,liberating water, and the vinyl portion of the molecule undergoespolymerization to form a highly crosslinked, thermally stable polymer.This particular system has drawbacks in that the oligomer is a solidwhich makes for a very "boardy" prepreg when composites are to befabricated since the resin system undergoes very little or no meltduring the processing cycle. This lack of melt necessitates the use ofextremely high pressure, for example more than 1000 psi, in order tofabricate consolidated composite parts. If the matrix resin weremeltable at reasonable temperatures, the pressure could be considerablyless and the process more readily useable.

There is thus a definite need in the art for a reduced temperaturepolymerization process for polyimide preparation.

Accordingly, an object of the present invention is provide an improvedprocess for preparing addition type polyimides that may be employed in a"hot melt" prepregging without the use of additional solvents.

Another object of the present invention is a process for reducing thepolymerization temperature of addition type polyimides.

Another object of the present invention is a novel polyimide having areduced melting temperature.

Another object of the present invention is a novel process of preparingan improved prepreg for use in fabricating composite structures.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, the foregoing and other objects areattained by employing a mixture of methylene dianilines as the diamineportion of the oligomer in an addition polymerization process to producean oligomer that is tacky and "nonboardy" when used to form a prepreg.This mixed diamine oligomer melts in the 175°-200° C. temperature rangeduring processing and requires only approximately 200 psi moldingpressure to fabricate composite structures that have equal or betterphysical property characteristics than those produced previously at 1000psi molding pressure.

Composites prepared from these oligomers can be thermoformed at elevatedtemperatures after an initial molding in the 175°-200° C. temperaturerange due to this lowered melting temperatures of the mixed diamines.

DETAILED DESCRIPTION

A more complete appreciation of the invention and many of the attendantadvantages thereof will be better understood when considered byreference to the following specific examples.

EXAMPLE 1

A mixture of 1.34 moles (432 g) of benzophenonetetracarboxylic aciddianhydride (BTDA) and 2.44 moles of nadic anhydride (NA), also known asnorbornene dicarboxylic acid anhydride, are heated to 75°-80° C. with a5.20 moles (plus a slight excess) of ethanol to form the diethylester ofBTDA (BTDE) and the ethylester of NA (NE). This mixture of BTDE and NEremain liquid when maintained at 40°-50° C.

This ester mixture is mixed with a mixture of amines having a totalamine content of 5.10-5.20 moles (504 g). This balance allows for laterreaction of the amines with the ester acids of BTDE and NE to form imiderings. The mixture of amines is composed of isomers ofmethylenedianilines (MDA) in essentially the following percentages:

o, o'--MDA--3.20%

o, p'--MDA--15.70%

p, p'--MDA--66.00%

Also included in the amine mixture are the tri and tetrafunctionalamines ##STR5##

The amine mixture remains liquid because the large number of isomers,all of which are solid at room temperature, tend to retardcrystallization because of the high degree of disorder, or large entropyfactor, which makes it difficult for nucleation and subsequentcrystallization to occur. This liquid mixture is further stabilized inthe liquid state when mixed with the ester mixture.

This mixture of monomers which can then be used without a carriersolvent for prepregging onto fibers allows for the formation of a tacky(or sticky) and drapeable, i.e. easy to contour, prepreg which is highlydesirable for composite or laminate fabrication in that complex shapesare easier to fabricate.

Other amine mixtures and amine-ester mixtures may also be utilized toform useful liquid laminating resins that are adaptable for "hot melt"prepregging onto fibers.

EXAMPLE II

To a 500 ml RB flask equipped with magnetic stirring are added 108 g(0.335 M) of BTDA and 100 g (0.610 M) of nadic anhydride. Next is added300 g of absolute ethanol and the mixture heated to 50°-60° C. untilsolution occurs, i.e. approximately 90 minutes. This mixture is cooledto room temperature and added to a vessel containing 126 g (0.539 M) ofJeffamine AP-22 (a tradename for a mixture of diamines sold by theJefferson Chemical Company, Inc., Houston, TX). This mixture was stirredfor approximately one hour at room temperature to obtain a homogeneouscondition.

    ______________________________________                                                            Functional Equivalent                                     ______________________________________                                        Nadic anhydride                                                                            0.6098 moles 0.6098                                              BTDA         0.3354 moles 0.6708                                                                        1.2806                                              ______________________________________                                    

This homogenous mixture may then be used for prepregging onto fibers asdescribed in Example I.

Example III

The BTDE (0.335 mole) and NE (0.610 mole) were dissolved in 200 g ofabsolute ethanol. To this was added the Jeffamine AP-22 (0.539 mole) andthe mixture stirred for one hour at room temperature to obtain ahomogenous mixture. Since the ester-acid result tends todisproportionate on standing, a characteristic of imide prepolymers, themixture should be refrigerated to extend the shelf life thereof. TheBTDE and NE may be obtained commercially from specialty chemicalcompanies or may be made by refluxing the stoichiometric amounts ofbenzophenonetetracarboxlic acid dianhydride (BTDA) and nadic anhydride(NA) with excess ethanol.

EXAMPLE IV

The homogeneous ester-acid imide prepolymer obtained in Example III washeated in the temperature range of 50°-70° C. to reduce the viscositythereof and brushed onto Hercules HT-S graphite fiber, drum wound at aspacing of 51/2 tows per inch, to yield prepreg with a fiber content of50% by weight. The prepreg was air-dried on the rotary drum forapproximately eight hours to reduce solvent content to approximately 9%by weight. At this point the material had drape, tack, and could beeasily cut and formed into shaped layups.

EXAMPLE V

The prepreg formed in Example IV was cut into 7.6 cm by 17.8 cm (3inch×7 inch) laminae and stacked into a 19-ply unidirectional preform.The preform billet for press molding was consolidated under simulatedautoclave conditions, using 7-14 kPa (2-4 inch Hg) vacuum. Each billetwas heated at a rate of 2.8° K. (5° F.) per minute to 436° K. (325° F.),held at this temperature for one hour and then cooled under vacuum toroom temperature. Weight loss during the consolidation and imidizationstep ranged between 25% and 29% and provided a well-formed billetaveraging 32% resin by weight. When billets were consolidated for fullautoclave molding, the curing schedule was continued uninterrupted afterone hour staging at 436° K. (325° F.).

The consolidated billets were trimmed to closely fit a flatmatched-metal mold having open ends in the 0° fiber direction whichpermitted observation of resin flow during molding. The matched mold waspreheated to 478° K. (400° F.) and charged with the billet, contactpressure was applied, and a heating rate of 2.8° K. (5° F.) per minuteestablished. At 546° K. (525° F.), pressure was increased to 1.4 MPa(200 psig) over a one-minute span, held for one minute, decreased tozero MPa (psig) for fifteen seconds (a bumping action), and thenreapplied at 1.4 MPa (200 psig). A maximum curing temperature of 589° K.(600° F.) was attained and held for two hours. The panel was cooled at2.8° K. (5° F.) per minute under full pressure and removed at atemperature below 366° K. (200° F.).

For panels completely autoclaved, B-staged billets were heated at 2.8°K. (5° F.) per minute to 589° K. (600° F.), with 1.4 MPa (200 psig)pressure applied at 547° K. (525° F.). After curing at 589° K. (600° F.)for two hours, the panels were cooled under autoclave pressure and fullvacuum on the bag at 2.8° K. (5° F.) per minute.

Pertinent properties of nine press molded panels are summarized asfollows:

(1) Resin flow during molding: 2.0±0.5% by weight

(2) Weight loss during molding: 0.6 to 1.0%

(3) Average panel thickness: 0.312 cm±0.005 cm (0.123 inch±0.002 inch)

(4) Resin content, by acid digestion: 30% by weight

(5) Panel density, by weight/dimension measurements: 1.61 gm/cm³

(6) Panel density, by displacement measurements: 1.55 gm/cm³

(7) Ultrasonic "C"--Scan measurements: void free

The autoclaved panels had an average thickness of 0.310 cm±0.010 cm(0.122 in.±0.004 in.), and experienced an average total weight lossduring molding of 30% by weight. The other properties listed above forthe press-molded panels were not measured for autoclaved panels.

The press molded composites of the present invention demonstrated thatthis resin system overcomes one major obstacle previously noted inprocessing polyimides, i.e. lack of flow. The imidized material of thepresent invention flowed readily when heated above 533° K. (500° F.). Ata heating rate of 2.8° K. (5° F.) the resin proceeded smoothly throughthe liquid state to a crosslinked system. In the region of full flow,the unrestrained resin evolved cyclopentadiene as determined by massspectral measurements. This has been noted for other addition typepolyimides that incorporate the nadic moiety. By pressurizing at 547° K.(525° F.) and continuing the processing cycle, the resin of the presentinvention was cured with substantially no loss of the reactioncyclopentadiene which subsequently copolymerized to the crosslinkedpolymer state. Also, post cure of a graphite reinforced panel at 589° K.(600° F.) for four hours in air caused no blistering and a weight lossof less than 0.1% suggesting that volatiles had not been entrappedduring the cure cycle.

EXAMPLE VI

The resin can also be prepared by using only a stoichiometric amount ofethanol, i.e. enough to only form the ester-acid. The nadic anhydride(200 g, 1.22 mole) and BTDA (216 g, 0.67 mole) are refluxed with astoichiometric amount of ethanol (151.1 g, 2.56 mole) in order to formthe ester-acid mixture. Thus, one mole of anhydride+one mole of ethanolyields one mole of ester-acid. A slight excess of ethanol (up to 5%) maybe added at this point in order to control the viscosity of the finalmix.

After solution of the ester-acid mixture has been accomplished, 2.56molar equivalents (252 g) of the amine mixture (as employed in theprevious Examples) is mixed in at approximately 50° C. This elevatedtemperature is necessary to keep the viscosity low and insure propermixing. This mixture is then cooled and may be stored for use inimpregnating fiber in the absence of additional solvent (solventlessprepregging) when heated to the temperature range of 50°-70° C. Thistechnique of prepregging is generally referred to as "hot melt"prepregging and has heretofore been confined to epoxy resins due to thehigh temperatures needed for melting the polyimides.

It is thus seen that the foregoing specific Examples are illustrative ofa novel addition polyimide system based on the use of liquid monomerswherein the essentially solventless prepreg produced therefrom retainsgood drape, tack, and other advantageous mechanical properties.

The specific Examples described herein are to merely illustrate theinvention and are not to be deemed as exhaustive. Thus, variousmodifications and variations of the present invention will be apparentto those skilled in the art without departing from the spirit and scopeof the appended claims.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method for preparing a novel addition typepolyimide prepreg comprising the steps of reacting equimolar quantitiesof benzophenonetetracarboxylic acid dianhydride and norbornenedicarboxylic acid anhydride in an excess of absolute ethanol whileheating to 50°-60° C. until solution occurs, cooling the resultingsolution of ethylesters to room temperature and adding the cooledsolution to a vessel containing an equimolar solution of mixeddianilines, stirring the combined solution for approximately one hour toobtain a homogeneous ester-acid polyimide prepolymer adaptable for"hot-melt" prepreg applications as a liquid resin system, and includingthe further steps of applying the liquid resin system onto fibers afterheating the resin to 50°-70° C. to lower the resin viscosity adqequatelyto permit prepregging in the absence of a carrier solvent and whereinthe mixed dianilines are the isomers of methylenedianilines and in thefollowing percentages:o,o'--MDA--3.20% o,p'--MDA--15.70%p,p'--MDA--66.00% ##STR6##
 2. The method of claim 1 including brushingthe heated ester-acid imide prepolymer onto drum-wound graphite fiber toyield a prepreg having a fiber content of approximately 52% by weight,air drying the drum-wound prepreg for approximately eight hours toreduce the solvent content to approximately 9% by weight and yield aprepreg having improved properties of drape, tack and adaptability forcutting and forming into shaped layers.
 3. The method of claim 2including the steps of cutting the air-dried prepreg into desiredlengths, stacking these lengths into a multiplied preform billet,heating the preformed billet under vacuum and press-mold pressure for afinal cure into a polyimide composite.
 4. The method of claim 3 whereinheating of the preformed billets is at a rate of 5° F. per minute to325° F. and maintained at approximately this temperature for a period ofone hour and thereafter heated at a rate of 5° F. per minute to 600° F.and maintained at this temperature for two hours.
 5. The method of claim4 wherein the press mold temperature is applied over a one minute spanwhen the billet temperature reaches 525° F. and at 200 psig, maintainedfor one minute, decreased to zero psig and then reapplied and maintainedat 200 psig during the remainder of the heating cycle.